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% \usepackage[square, comma, sort&compress]{natbib} |
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9.0in \textwidth 6.5in \brokenpenalty=1110000 |
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% double space list of tables and figures |
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%\AtBeginDelayedFloats{\renewcomand{\baselinestretch}{1.66}} |
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\begin{abstract} |
| 79 |
< |
We examine surface reconstructions of Pt and Au(557) under |
| 80 |
< |
various CO coverages using molecular dynamics in order to |
| 81 |
< |
explore possible mechanisms for any observed reconstructions |
| 82 |
< |
and their dynamics. The metal-CO interactions were parameterized |
| 83 |
< |
as part of this work so that an efficient large-scale treatment of |
| 84 |
< |
this system could be undertaken. The large difference in binding |
| 85 |
< |
strengths of the metal-CO interactions was found to play a significant |
| 86 |
< |
role with regards to step-edge stability and adatom diffusion. A |
| 87 |
< |
small correlation between coverage and the diffusion constant |
| 88 |
< |
was also determined. The energetics of CO adsorbed to the surface |
| 89 |
< |
is sufficient to explain the reconstructions observed on the Pt |
| 90 |
< |
systems and the lack of reconstruction of the Au systems. |
| 88 |
< |
|
| 89 |
< |
|
| 90 |
< |
The mechanism and dynamics of surface reconstructions of Pt(557) |
| 91 |
< |
and Au(557) exposed to various coverages of carbon monoxide (CO) |
| 92 |
< |
were investigated using molecular dynamics simulations. Metal-CO |
| 93 |
< |
interactions were parameterized from experimental data and plane-wave |
| 94 |
< |
Density Functional Theory (DFT) calculations. The large difference in |
| 95 |
< |
binding strengths of the Pt-CO and Au-CO interactions was found to play |
| 96 |
< |
a significant role in step-edge stability and adatom diffusion constants. |
| 97 |
< |
The energetics of CO adsorbed to the surface is sufficient to explain the |
| 98 |
< |
step-doubling reconstruction observed on Pt(557) and the lack of such |
| 99 |
< |
a reconstruction on the Au(557) surface. |
| 79 |
> |
The mechanism and dynamics of surface reconstructions of Pt(557) and |
| 80 |
> |
Au(557) exposed to various coverages of carbon monoxide (CO) were |
| 81 |
> |
investigated using molecular dynamics simulations. Metal-CO |
| 82 |
> |
interactions were parameterized from experimental data and |
| 83 |
> |
plane-wave Density Functional Theory (DFT) calculations. The large |
| 84 |
> |
difference in binding strengths of the Pt-CO and Au-CO interactions |
| 85 |
> |
was found to play a significant role in step-edge stability and |
| 86 |
> |
adatom diffusion constants. Various mechanisms for CO-mediated step |
| 87 |
> |
wandering and step doubling were investigated on the Pt(557) |
| 88 |
> |
surface. We find that the energetics of CO adsorbed to the surface |
| 89 |
> |
can explain the step-doubling reconstruction observed on Pt(557) and |
| 90 |
> |
the lack of such a reconstruction on the Au(557) surface. |
| 91 |
|
\end{abstract} |
| 92 |
|
|
| 93 |
|
\newpage |
| 209 |
|
The EAM, Finnis-Sinclair, and the Quantum Sutton-Chen (QSC) potentials |
| 210 |
|
have all been widely used by the materials simulation community for |
| 211 |
|
simulations of bulk and nanoparticle |
| 212 |
< |
properties,\cite{Chui:2003fk,Wang:2005qy,Medasani:2007uq} |
| 212 |
> |
properties,\cite{Chui:2003fk,Wang:2005qy,Medasani:2007uq,mishin99:_inter} |
| 213 |
|
melting,\cite{Belonoshko00,sankaranarayanan:155441,Sankaranarayanan:2005lr} |
| 214 |
< |
fracture,\cite{Shastry:1996qg,Shastry:1998dx} crack |
| 215 |
< |
propagation,\cite{BECQUART:1993rg} and alloying |
| 216 |
< |
dynamics.\cite{Shibata:2002hh} One of EAM's strengths |
| 214 |
> |
fracture,\cite{Shastry:1996qg,Shastry:1998dx,mishin01:cu} crack |
| 215 |
> |
propagation,\cite{BECQUART:1993rg,Rifkin1992} and alloying |
| 216 |
> |
dynamics.\cite{Shibata:2002hh,mishin02:b2nial,zope03:tial_ap,mishin05:phase_fe_ni} One of EAM's strengths |
| 217 |
|
is its sensitivity to small changes in structure. This arises |
| 218 |
|
because interactions |
| 219 |
|
up to the third nearest neighbor were taken into account in the parameterization.\cite{Voter95a} |
| 224 |
|
surface structures. Additionally, the similarity of EAM's functional |
| 225 |
|
treatment of the embedding energy to standard density functional |
| 226 |
|
theory (DFT) makes fitting DFT-derived cross potentials with adsorbates somewhat easier. |
| 227 |
< |
\cite{Foiles86,PhysRevB.37.3924,Rifkin1992,mishin99:_inter,mishin01:cu,mishin02:b2nial,zope03:tial_ap,mishin05:phase_fe_ni} |
| 227 |
> |
|
| 228 |
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|
| 229 |
|
|
| 230 |
|
|
| 390 |
|
data collection. All of the systems examined had at least 40~ns in the |
| 391 |
|
data collection stage, although simulation times for some Pt of the |
| 392 |
|
systems exceeded 200~ns. Simulations were carried out using the open |
| 393 |
< |
source molecular dynamics package, OpenMD.\cite{Ewald,OOPSE} |
| 393 |
> |
source molecular dynamics package, OpenMD.\cite{Ewald,OOPSE,openmd} |
| 394 |
|
|
| 395 |
|
|
| 396 |
|
|
| 462 |
|
|
| 463 |
|
%Evolution of surface |
| 464 |
|
\begin{figure}[H] |
| 465 |
< |
\includegraphics[width=\linewidth]{ProgressionOfDoubleLayerFormation_yellowCircle.png} |
| 465 |
> |
\includegraphics[width=\linewidth]{EPS_ProgressionOfDoubleLayerFormation} |
| 466 |
|
\caption{The Pt(557) / 50\% CO system at a sequence of times after |
| 467 |
|
initial exposure to the CO: (a) 258~ps, (b) 19~ns, (c) 31.2~ns, and |
| 468 |
|
(d) 86.1~ns. Disruption of the (557) step-edges occurs quickly. The |
| 521 |
|
|
| 522 |
|
%Diffusion graph |
| 523 |
|
\begin{figure}[H] |
| 524 |
< |
\includegraphics[width=\linewidth]{DiffusionComparison_errorXY_remade_20ns.pdf} |
| 524 |
> |
\includegraphics[width=\linewidth]{Portrait_DiffusionComparison_1} |
| 525 |
|
\caption{Diffusion constants for mobile surface atoms along directions |
| 526 |
|
parallel ($\mathbf{D}_{\parallel}$) and perpendicular |
| 527 |
|
($\mathbf{D}_{\perp}$) to the (557) step-edges as a function of CO |
| 577 |
|
|
| 578 |
|
%Discussion |
| 579 |
|
\section{Discussion} |
| 580 |
< |
We have shown that a classical potential model is able to model the |
| 581 |
< |
initial reconstruction of the Pt(557) surface upon CO adsorption as |
| 582 |
< |
shown by Tao {\it et al}.\cite{Tao:2010}. More importantly, we were |
| 583 |
< |
able to observe features of the dynamic processes necessary for |
| 584 |
< |
this reconstruction. Here we discuss the features of the model that |
| 585 |
< |
give rise to the observed dynamical properties of the (557) reconstruction. |
| 580 |
> |
We have shown that a classical potential is able to model the initial |
| 581 |
> |
reconstruction of the Pt(557) surface upon CO adsorption, and have |
| 582 |
> |
reproduced the double layer structure observed by Tao {\it et |
| 583 |
> |
al}.\cite{Tao:2010}. Additionally, this reconstruction appears to be |
| 584 |
> |
rapid -- occurring within 100 ns of the initial exposure to CO. Here |
| 585 |
> |
we discuss the features of the classical potential that are |
| 586 |
> |
contributing to the stability and speed of the Pt(557) reconstruction. |
| 587 |
|
|
| 588 |
|
\subsection{Diffusion} |
| 589 |
< |
The perpendicular diffusion constant |
| 590 |
< |
appears to be the most important indicator of double layer |
| 591 |
< |
formation. As highlighted in Figure \ref{fig:reconstruct}, the |
| 592 |
< |
formation of the double layer did not begin until a nucleation |
| 593 |
< |
site appeared. And as mentioned by Williams {\it et al}.\cite{Williams:1991, Williams:1994}, |
| 594 |
< |
the inability for edges to cross leads to an effective edge-edge repulsion that |
| 595 |
< |
must be overcome to allow step coalescence. |
| 596 |
< |
A greater $\textbf{D}_\perp$ implies more step-wandering |
| 597 |
< |
and a larger chance for the stochastic meeting of two edges |
| 598 |
< |
to create a nucleation point. Parallel diffusion along the step-edge can help ``zipper'' up a nascent double |
| 599 |
< |
layer. This helps explain why the time scale for formation after |
| 600 |
< |
the appearance of a nucleation site was rapid, while the initial |
| 601 |
< |
appearance of the nucleation site was unpredictable. |
| 589 |
> |
The perpendicular diffusion constant appears to be the most important |
| 590 |
> |
indicator of double layer formation. As highlighted in Figure |
| 591 |
> |
\ref{fig:reconstruct}, the formation of the double layer did not begin |
| 592 |
> |
until a nucleation site appeared. Williams {\it et |
| 593 |
> |
al}.\cite{Williams:1991,Williams:1994} cite an effective edge-edge |
| 594 |
> |
repulsion arising from the inability of edge crossing. This repulsion |
| 595 |
> |
must be overcome to allow step coalescence. A larger |
| 596 |
> |
$\textbf{D}_\perp$ value implies more step-wandering and a larger |
| 597 |
> |
chance for the stochastic meeting of two edges to create a nucleation |
| 598 |
> |
point. Diffusion parallel to the step-edge can help ``zipper'' up a |
| 599 |
> |
nascent double layer. This helps explain the rapid time scale for |
| 600 |
> |
double layer completion after the appearance of a nucleation site, while |
| 601 |
> |
the initial appearance of the nucleation site was unpredictable. |
| 602 |
|
|
| 603 |
|
\subsection{Mechanism for restructuring} |
| 604 |
< |
Since the Au surface showed no large scale restructuring in any of |
| 605 |
< |
our simulations, our discussion will focus on the 50\% Pt-CO system |
| 606 |
< |
which did exhibit doubling. A |
| 607 |
< |
number of possible mechanisms exist to explain the role of adsorbed |
| 608 |
< |
CO in restructuring the Pt surface. Quadrupolar repulsion between |
| 609 |
< |
adjacent CO molecules adsorbed on the surface is one possibility. |
| 610 |
< |
However, the quadrupole-quadrupole interaction is short-ranged and |
| 611 |
< |
is attractive for some orientations. If the CO molecules are ``locked'' in |
| 612 |
< |
a specific orientation relative to each other, through atop adsorption for |
| 613 |
< |
example, this explanation would gain credence. The calculated energetic repulsion |
| 614 |
< |
between two CO molecules located a distance of 2.77~\AA~apart |
| 615 |
< |
(nearest-neighbor distance of Pt) and both in a vertical orientation, |
| 616 |
< |
is 8.62 kcal/mol. Moving the CO to the second nearest-neighbor distance |
| 617 |
< |
of 4.8~\AA~drops the repulsion to nearly 0. Allowing the CO to rotate away |
| 618 |
< |
from a purely vertical orientation also lowers the repulsion. When the |
| 619 |
< |
carbons are locked at a distance of 2.77~\AA, a minimum of 6.2 kcal/mol is |
| 620 |
< |
reached when the angle between the 2 CO is $\sim$24\textsuperscript{o}. |
| 621 |
< |
The calculated barrier for surface diffusion of a Pt adatom is only 4 kcal/mol, so |
| 622 |
< |
repulsion between adjacent CO molecules bound to Pt could increase the surface |
| 623 |
< |
diffusion. However, the residence time of CO on Pt suggests that these |
| 624 |
< |
molecules are extremely mobile, with diffusion constants 40 to 2500 times |
| 625 |
< |
larger than surface Pt atoms. This mobility suggests that the CO molecules jump |
| 626 |
< |
between different Pt atoms throughout the simulation, but will stay bound for |
| 627 |
< |
significant periods of time. |
| 604 |
> |
Since the Au surface showed no large scale restructuring in any of our |
| 605 |
> |
simulations, our discussion will focus on the 50\% Pt-CO system which |
| 606 |
> |
did exhibit doubling. A number of possible mechanisms exist to explain |
| 607 |
> |
the role of adsorbed CO in restructuring the Pt surface. Quadrupolar |
| 608 |
> |
repulsion between adjacent CO molecules adsorbed on the surface is one |
| 609 |
> |
possibility. However, the quadrupole-quadrupole interaction is |
| 610 |
> |
short-ranged and is attractive for some orientations. If the CO |
| 611 |
> |
molecules are ``locked'' in a vertical orientation, through atop |
| 612 |
> |
adsorption for example, this explanation would gain credence. The |
| 613 |
> |
calculated energetic repulsion between two CO molecules located a |
| 614 |
> |
distance of 2.77~\AA~apart (nearest-neighbor distance of Pt) and both |
| 615 |
> |
in a vertical orientation, is 8.62 kcal/mol. Moving the CO to the |
| 616 |
> |
second nearest-neighbor distance of 4.8~\AA~drops the repulsion to |
| 617 |
> |
nearly 0. Allowing the CO to rotate away from a purely vertical |
| 618 |
> |
orientation also lowers the repulsion. When the carbons are locked at |
| 619 |
> |
a distance of 2.77~\AA, a minimum of 6.2 kcal/mol is reached when the |
| 620 |
> |
angle between the 2 CO is $\sim$24\textsuperscript{o}. The calculated |
| 621 |
> |
barrier for surface diffusion of a Pt adatom is only 4 kcal/mol, so |
| 622 |
> |
repulsion between adjacent CO molecules bound to Pt could increase the |
| 623 |
> |
surface diffusion. However, the residence time of CO on Pt suggests |
| 624 |
> |
that the CO molecules are extremely mobile, with diffusion constants 40 |
| 625 |
> |
to 2500 times larger than surface Pt atoms. This mobility suggests |
| 626 |
> |
that the CO molecules jump between different Pt atoms throughout the |
| 627 |
> |
simulation, but can stay bound for significant periods of time. |
| 628 |
|
|
| 629 |
< |
A different interpretation of the above mechanism, taking into account the large |
| 630 |
< |
mobility of the CO, looks at how instantaneous and short-lived configurations of |
| 631 |
< |
CO on the surface can destabilize Pt-Pt interactions leading to increased step-edge |
| 632 |
< |
breakup and diffusion. On the bare Pt(557) surface the barrier to completely detach |
| 633 |
< |
an edge atom is $\sim$43~kcal/mol, as is shown in configuration (a) in Figures |
| 634 |
< |
\ref{fig:SketchGraphic} \& \ref{fig:SketchEnergies}. For certain configurations, cases |
| 635 |
< |
(e), (g), and (h), the barrier can be lowered to $\sim$23~kcal/mole. In these instances, |
| 636 |
< |
it becomes quite energetically favorable to roughen the edge by introducing a small |
| 637 |
< |
separation of 0.5 to 1.0~\AA. This roughening becomes immediately obvious in |
| 638 |
< |
simulations with significant CO populations. The roughening is present to a lesser extent |
| 639 |
< |
on lower coverage surfaces and even on the bare surfaces, although in these cases it is likely |
| 640 |
< |
due to stochastic vibrational processes that squeeze out step-edge atoms. The mechanism |
| 641 |
< |
of step-edge breakup suggested by these energy curves is one of the most difficult |
| 642 |
< |
processes, a complete break-away from the step-edge in one unbroken movement. |
| 643 |
< |
Easier multistep mechanisms likely exist where an adatom moves laterally on the surface |
| 644 |
< |
after being ejected so it ends up alongside the ledge. This provides the atom with 5 nearest |
| 645 |
< |
neighbors, which while lower than the 7 if it had stayed a part of the step-edge, is higher |
| 646 |
< |
than the 3 it could maintain located on the terrace. In this proposed mechanism, the CO |
| 647 |
< |
quadrupolar repulsion is still playing a primary role, but for its importance in roughening |
| 648 |
< |
the step-edge, rather than maintaining long-term bonds with a single Pt atom which is not |
| 649 |
< |
born out by their mobility data. The requirement for a large density of CO on the surface |
| 650 |
< |
for some of the more favorable suggested configurations in Figure \ref{fig:SketchGraphic} |
| 651 |
< |
correspond well with the increased mobility seen on higher coverage surfaces. |
| 629 |
> |
A different interpretation of the above mechanism which takes the |
| 630 |
> |
large mobility of the CO into account, would be in the destabilization |
| 631 |
> |
of Pt-Pt interactions due to bound CO. Destabilizing Pt-Pt bonds at |
| 632 |
> |
the edges could lead to increased step-edge breakup and diffusion. On |
| 633 |
> |
the bare Pt(557) surface the barrier to completely detach an edge atom |
| 634 |
> |
is $\sim$43~kcal/mol, as is shown in configuration (a) in Figures |
| 635 |
> |
\ref{fig:SketchGraphic} \& \ref{fig:SketchEnergies}. For certain |
| 636 |
> |
configurations, cases (e), (g), and (h), the barrier can be lowered to |
| 637 |
> |
$\sim$23~kcal/mol by the presence of bound CO molecules. In these |
| 638 |
> |
instances, it becomes energetically favorable to roughen the edge by |
| 639 |
> |
introducing a small separation of 0.5 to 1.0~\AA. This roughening |
| 640 |
> |
becomes immediately obvious in simulations with significant CO |
| 641 |
> |
populations. The roughening is present to a lesser extent on surfaces |
| 642 |
> |
with lower CO coverage (and even on the bare surfaces), although in |
| 643 |
> |
these cases it is likely due to random fluctuations that squeeze out |
| 644 |
> |
step-edge atoms. Step-edge breakup by continuous single-atom |
| 645 |
> |
translations (as suggested by these energy curves) is probably a |
| 646 |
> |
worst-case scenario. Multistep mechanisms in which an adatom moves |
| 647 |
> |
laterally on the surface after being ejected would be more |
| 648 |
> |
energetically favorable. This would leave the adatom alongside the |
| 649 |
> |
ledge, providing it with 5 nearest neighbors. While fewer than the 7 |
| 650 |
> |
neighbors it had as part of the step-edge, it keeps more Pt neighbors |
| 651 |
> |
than the 3 an isolated adatom would have on the terrace. In this |
| 652 |
> |
proposed mechanism, the CO quadrupolar repulsion still plays a role in |
| 653 |
> |
the initial roughening of the step-edge, but not in any long-term |
| 654 |
> |
bonds with individual Pt atoms. Higher CO coverages create more |
| 655 |
> |
opportunities for the crowded CO configurations shown in Figure |
| 656 |
> |
\ref{fig:SketchGraphic}, and this is likely to cause an increased |
| 657 |
> |
propensity for step-edge breakup. |
| 658 |
|
|
| 659 |
|
%Sketch graphic of different configurations |
| 660 |
|
\begin{figure}[H] |
| 661 |
< |
\includegraphics[width=0.8\linewidth, height=0.8\textheight]{COpathsSketch.pdf} |
| 662 |
< |
\caption{The dark grey atoms refer to the upper ledge, while the white atoms are |
| 663 |
< |
the lower terrace. The blue highlighted atoms had a CO in a vertical atop position |
| 664 |
< |
upon them. These are a sampling of the configurations examined to gain a more |
| 665 |
< |
complete understanding of the effects CO has on surface diffusion and edge breakup. |
| 666 |
< |
Energies associated with each configuration are displayed in Figure \ref{fig:SketchEnergies}.} |
| 661 |
> |
\includegraphics[width=\linewidth]{COpaths} |
| 662 |
> |
\caption{Configurations used to investigate the mechanism of step-edge |
| 663 |
> |
breakup on Pt(557). In each case, the central (starred) atom is |
| 664 |
> |
pulled directly across the surface away from the step edge. The Pt |
| 665 |
> |
atoms on the upper terrace are colored dark grey, while those on the |
| 666 |
> |
lower terrace are in white. In each of these configurations, some |
| 667 |
> |
number of the atoms (highlighted in blue) had a CO molecule bound in |
| 668 |
> |
a vertical atop position. The energies of these configurations as a |
| 669 |
> |
function of central atom displacement are displayed in Figure |
| 670 |
> |
\ref{fig:SketchEnergies}.} |
| 671 |
|
\label{fig:SketchGraphic} |
| 672 |
|
\end{figure} |
| 673 |
|
|
| 674 |
|
%energy graph corresponding to sketch graphic |
| 675 |
|
\begin{figure}[H] |
| 676 |
< |
\includegraphics[width=\linewidth]{stepSeparationComparison.pdf} |
| 677 |
< |
\caption{The energy curves directly correspond to the labeled model |
| 678 |
< |
surface in Figure \ref{fig:SketchGraphic}. All energy curves are relative |
| 679 |
< |
to their initial configuration so the energy of a and h do not have the |
| 680 |
< |
same zero value. As is seen, certain arrangements of CO can lower |
| 681 |
< |
the energetic barrier that must be overcome to create an adatom. |
| 682 |
< |
However, it is the highest coverages where these higher-energy |
| 683 |
< |
configurations of CO will be more likely. } |
| 676 |
> |
\includegraphics[width=\linewidth]{Portrait_SeparationComparison} |
| 677 |
> |
\caption{Energies for displacing a single edge atom perpendicular to |
| 678 |
> |
the step edge as a function of atomic displacement. Each of the |
| 679 |
> |
energy curves corresponds to one of the labeled configurations in |
| 680 |
> |
Figure \ref{fig:SketchGraphic}, and are referenced to the |
| 681 |
> |
unperturbed step-edge. Certain arrangements of bound CO (notably |
| 682 |
> |
configurations g and h) can lower the energetic barrier for creating |
| 683 |
> |
an adatom relative to the bare surface (configuration a).} |
| 684 |
|
\label{fig:SketchEnergies} |
| 685 |
|
\end{figure} |
| 686 |
|
|
| 687 |
< |
While configurations of CO on the surface are able to increase diffusion, |
| 688 |
< |
this does not immediately provide an explanation for the formation of double |
| 689 |
< |
layers. If adatoms were constrained to their terrace then doubling would be |
| 690 |
< |
much less likely to occur. Nucleation sites could still potentially form, but there |
| 691 |
< |
would not be enough atoms to finish the doubling. For a non-simulated metal surface, where the |
| 692 |
< |
step lengths can be assumed to be infinite relative to atomic sizes, local doubling would be possible, but in |
| 691 |
< |
our simulations with our periodic treatment of the system, the system is not large enough to experience this effect. |
| 692 |
< |
Thus, there must be a mechanism that explains how adatoms are able to move |
| 693 |
< |
amongst terraces. Figure \ref{fig:lambda} shows points along a reaction coordinate |
| 694 |
< |
where an adatom along the step-edge with an adsorbed CO ``burrows'' into the |
| 695 |
< |
edge displacing an atom onto the higher terrace. This mechanism was chosen |
| 696 |
< |
because of similar events that were observed during the simulations. The barrier |
| 697 |
< |
heights we obtained are only approximations because we constrained the movement |
| 698 |
< |
of the highlighted atoms along a specific concerted path. The calculated $\Delta E$'s |
| 699 |
< |
are provide a strong energetic support for this modeled lifting mechanism. When CO is not present and |
| 700 |
< |
this reaction coordinate is followed, the $\Delta E > 3$~kcal/mol. The example shown |
| 701 |
< |
in the figure, where CO is present in the atop position, has a $\Delta E < -15$~kcal/mol. |
| 702 |
< |
While the barrier height is comparable for both cases, there is nearly a 20~kcal/mol |
| 703 |
< |
difference in energies and makes the process energetically favorable. |
| 687 |
> |
While configurations of CO on the surface are able to increase |
| 688 |
> |
diffusion and the likelihood of edge wandering, this does not provide |
| 689 |
> |
a complete explanation for the formation of double layers. If adatoms |
| 690 |
> |
were constrained to their original terraces then doubling could not |
| 691 |
> |
occur. A mechanism for vertical displacement of adatoms at the |
| 692 |
> |
step-edge is required to explain the doubling. |
| 693 |
|
|
| 694 |
+ |
We have discovered one possible mechanism for a CO-mediated vertical |
| 695 |
+ |
displacement of Pt atoms at the step edge. Figure \ref{fig:lambda} |
| 696 |
+ |
shows four points along a reaction coordinate in which a CO-bound |
| 697 |
+ |
adatom along the step-edge ``burrows'' into the edge and displaces the |
| 698 |
+ |
original edge atom onto the higher terrace. A number of events similar |
| 699 |
+ |
to this mechanism were observed during the simulations. We predict an |
| 700 |
+ |
energetic barrier of 20~kcal/mol for this process (in which the |
| 701 |
+ |
displaced edge atom follows a curvilinear path into an adjacent 3-fold |
| 702 |
+ |
hollow site). The barrier heights we obtain for this reaction |
| 703 |
+ |
coordinate are approximate because the exact path is unknown, but the |
| 704 |
+ |
calculated energy barriers would be easily accessible at operating |
| 705 |
+ |
conditions. Additionally, this mechanism is exothermic, with a final |
| 706 |
+ |
energy 15~kcal/mol below the original $\lambda = 0$ configuration. |
| 707 |
+ |
When CO is not present and this reaction coordinate is followed, the |
| 708 |
+ |
process is endothermic by 3~kcal/mol. The difference in the relative |
| 709 |
+ |
energies for the $\lambda=0$ and $\lambda=1$ case when CO is present |
| 710 |
+ |
provides strong support for CO-mediated Pt-Pt interactions giving rise |
| 711 |
+ |
to the doubling reconstruction. |
| 712 |
+ |
|
| 713 |
|
%lambda progression of Pt -> shoving its way into the step |
| 714 |
|
\begin{figure}[H] |
| 715 |
< |
\includegraphics[width=\linewidth]{lambdaProgression_atopCO_withLambda.png} |
| 716 |
< |
\caption{ Various points along a reaction coordinate are displayed in the figure. |
| 717 |
< |
The mechanism of edge traversal is examined in the presence of CO. The approximate |
| 718 |
< |
barrier for the displayed process is 20~kcal/mol. However, the $\Delta E$ of this process |
| 719 |
< |
is -15~kcal/mol making it an energetically favorable process.} |
| 715 |
> |
\includegraphics[width=\linewidth]{EPS_rxnCoord} |
| 716 |
> |
\caption{Points along a possible reaction coordinate for CO-mediated |
| 717 |
> |
edge doubling. Here, a CO-bound adatom burrows into an established |
| 718 |
> |
step edge and displaces an edge atom onto the upper terrace along a |
| 719 |
> |
curvilinear path. The approximate barrier for the process is |
| 720 |
> |
20~kcal/mol, and the complete process is exothermic by 15~kcal/mol |
| 721 |
> |
in the presence of CO, but is endothermic by 3~kcal/mol without.} |
| 722 |
|
\label{fig:lambda} |
| 723 |
|
\end{figure} |
| 724 |
|
|
| 725 |
< |
The mechanism for doubling on this surface appears to require the cooperation of at least |
| 726 |
< |
these two described processes. For complete doubling of a layer to occur there must |
| 727 |
< |
be the equivalent removal of a separate terrace. For those atoms to ``disappear'' from |
| 728 |
< |
that terrace they must either rise up on the ledge above them or drop to the ledge below |
| 729 |
< |
them. The presence of CO helps with the energetics of both of these situations. There must be sufficient |
| 730 |
< |
breakage of the step-edge to increase the concentration of adatoms on the surface and |
| 731 |
< |
these adatoms must then undergo the burrowing highlighted above or some comparable |
| 732 |
< |
mechanism to traverse the step-edge. Over time, these mechanisms working in concert |
| 733 |
< |
lead to the formation of a double layer. |
| 725 |
> |
The mechanism for doubling on the Pt(557) surface appears to require |
| 726 |
> |
the cooperation of at least two distinct processes. For complete |
| 727 |
> |
doubling of a layer to occur there must be a breakup of one |
| 728 |
> |
terrace. These atoms must then ``disappear'' from that terrace, either |
| 729 |
> |
by travelling to the terraces above of below their original levels. |
| 730 |
> |
The presence of CO helps explain mechanisms for both of these |
| 731 |
> |
situations. There must be sufficient breakage of the step-edge to |
| 732 |
> |
increase the concentration of adatoms on the surface and these adatoms |
| 733 |
> |
must then undergo the burrowing highlighted above (or a comparable |
| 734 |
> |
mechanism) to create the double layer. With sufficient time, these |
| 735 |
> |
mechanisms working in concert lead to the formation of a double layer. |
| 736 |
|
|
| 737 |
|
\subsection{CO Removal and double layer stability} |
| 738 |
< |
Once a double layer had formed on the 50\%~Pt system it |
| 739 |
< |
remained for the rest of the simulation time with minimal |
| 740 |
< |
movement. There were configurations that showed small |
| 741 |
< |
wells or peaks forming, but typically within a few nanoseconds |
| 742 |
< |
the feature would smooth away. Within our simulation time, |
| 743 |
< |
the formation of the double layer was irreversible and a double |
| 744 |
< |
layer was never observed to split back into two single layer |
| 733 |
< |
step-edges while CO was present. To further gauge the effect |
| 734 |
< |
CO had on this system, additional simulations were run starting |
| 735 |
< |
from a late configuration of the 50\%~Pt system that had formed |
| 736 |
< |
double layers. These simulations then had their CO removed. |
| 737 |
< |
The double layer breaks rapidly in these simulations, already |
| 738 |
< |
showing a well-defined splitting after 100~ps. Configurations of |
| 739 |
< |
this system are shown in Figure \ref{fig:breaking}. The coloring |
| 740 |
< |
of the top and bottom layers helps to exhibit how much mixing |
| 741 |
< |
the edges experience as they split. These systems were only |
| 742 |
< |
examined briefly, 10~ns, and within that time despite the initial |
| 743 |
< |
rapid splitting, the edges only moved another few \AA~apart. |
| 744 |
< |
It is possible with longer simulation times that the |
| 745 |
< |
(557) lattice could be recovered as seen by Tao {\it et al}.\cite{Tao:2010} |
| 738 |
> |
Once a double layer had formed on the 50\%~Pt system, it remained for |
| 739 |
> |
the rest of the simulation time with minimal movement. Random |
| 740 |
> |
fluctuations that involved small clusters or divots were observed, but |
| 741 |
> |
these features typically healed within a few nanoseconds. Within our |
| 742 |
> |
simulations, the formation of the double layer appeared to be |
| 743 |
> |
irreversible and a double layer was never observed to split back into |
| 744 |
> |
two single layer step-edges while CO was present. |
| 745 |
|
|
| 746 |
+ |
To further gauge the effect CO has on this surface, additional |
| 747 |
+ |
simulations were run starting from a late configuration of the 50\%~Pt |
| 748 |
+ |
system that had already formed double layers. These simulations then |
| 749 |
+ |
had their CO forcibly removed. The double layer broke apart rapidly |
| 750 |
+ |
in these simulations, showing a well-defined edge-splitting after |
| 751 |
+ |
100~ps. Configurations of this system are shown in Figure |
| 752 |
+ |
\ref{fig:breaking}. The coloring of the top and bottom layers helps to |
| 753 |
+ |
exhibit how much mixing the edges experience as they split. These |
| 754 |
+ |
systems were only examined for 10~ns, and within that time despite the |
| 755 |
+ |
initial rapid splitting, the edges only moved another few |
| 756 |
+ |
\AA~apart. It is possible that with longer simulation times, the (557) |
| 757 |
+ |
surface recovery observed by Tao {\it et al}.\cite{Tao:2010} could |
| 758 |
+ |
also be recovered. |
| 759 |
|
|
| 748 |
– |
|
| 760 |
|
%breaking of the double layer upon removal of CO |
| 761 |
|
\begin{figure}[H] |
| 762 |
< |
\includegraphics[width=\linewidth]{doubleLayerBreaking_greenBlue_whiteLetters.png} |
| 763 |
< |
\caption{(A) 0~ps, (B) 100~ps, (C) 1~ns, after the removal of CO. The presence of the CO |
| 764 |
< |
helped maintain the stability of the double layer and its microfaceting of the double layer |
| 765 |
< |
into a (111) configuration. This microfacet immediately reverts to the original (100) step |
| 766 |
< |
edge which is a hallmark of the (557) surface. The separation is not a simple sliding apart, rather |
| 767 |
< |
there is a mixing of the lower and upper atoms at the edge.} |
| 762 |
> |
\includegraphics[width=\linewidth]{EPS_doubleLayerBreaking} |
| 763 |
> |
\caption{Dynamics of an established (111) double step after removal of |
| 764 |
> |
the adsorbed CO: (A) 0~ps, (B) 100~ps, and (C) 1~ns after the removal |
| 765 |
> |
of CO. The presence of the CO helped maintain the stability of the |
| 766 |
> |
double step. Nearly immediately after the CO is removed, the step |
| 767 |
> |
edge reforms in a (100) configuration, which is also the step type |
| 768 |
> |
seen on clean (557) surfaces. The step separation involves |
| 769 |
> |
significant mixing of the lower and upper atoms at the edge.} |
| 770 |
|
\label{fig:breaking} |
| 771 |
|
\end{figure} |
| 772 |
|
|
| 773 |
|
|
| 761 |
– |
|
| 762 |
– |
|
| 774 |
|
%Peaks! |
| 775 |
|
%\begin{figure}[H] |
| 776 |
|
%\includegraphics[width=\linewidth]{doublePeaks_noCO.png} |
| 784 |
|
%Don't think I need this |
| 785 |
|
%clean surface... |
| 786 |
|
%\begin{figure}[H] |
| 787 |
< |
%\includegraphics[width=\linewidth]{557_300K_cleanPDF.pdf} |
| 787 |
> |
%\includegraphics[width=\linewidth]{557_300K_cleanPDF} |
| 788 |
|
%\caption{} |
| 789 |
|
|
| 790 |
|
%\end{figure} |
| 792 |
|
|
| 793 |
|
|
| 794 |
|
\section{Conclusion} |
| 795 |
< |
The strength of the Pt-CO binding interaction as well as the large |
| 796 |
< |
quadrupolar repulsion between CO molecules are sufficient to |
| 797 |
< |
explain the observed increase in surface mobility and the resultant |
| 798 |
< |
reconstructions at the highest simulated coverage. The weaker |
| 799 |
< |
Au-CO interaction results in lower diffusion constants, less step-wandering, |
| 800 |
< |
and a lack of the double layer reconstruction. An in-depth examination |
| 801 |
< |
of the energetics shows the important role CO plays in increasing |
| 802 |
< |
step-breakup and in facilitating edge traversal which are both |
| 792 |
< |
necessary for double layer formation. |
| 795 |
> |
The strength and directionality of the Pt-CO binding interaction, as |
| 796 |
> |
well as the large quadrupolar repulsion between atop-bound CO |
| 797 |
> |
molecules, help to explain the observed increase in surface mobility |
| 798 |
> |
of Pt(557) and the resultant reconstruction into a double-layer |
| 799 |
> |
configuration at the highest simulated CO-coverages. The weaker Au-CO |
| 800 |
> |
interaction results in significantly lower adataom diffusion |
| 801 |
> |
constants, less step-wandering, and a lack of the double layer |
| 802 |
> |
reconstruction on the Au(557) surface. |
| 803 |
|
|
| 804 |
+ |
An in-depth examination of the energetics shows the important role CO |
| 805 |
+ |
plays in increasing step-breakup and in facilitating edge traversal |
| 806 |
+ |
which are both necessary for double layer formation. |
| 807 |
|
|
| 795 |
– |
|
| 808 |
|
%Things I am not ready to remove yet |
| 809 |
|
|
| 810 |
|
%Table of Diffusion Constants |
| 828 |
|
% \end{table} |
| 829 |
|
|
| 830 |
|
\begin{acknowledgement} |
| 831 |
< |
Support for this project was provided by the National Science |
| 832 |
< |
Foundation under grant CHE-0848243 and by the Center for Sustainable |
| 833 |
< |
Energy at Notre Dame (cSEND). Computational time was provided by the |
| 834 |
< |
Center for Research Computing (CRC) at the University of Notre Dame. |
| 831 |
> |
We gratefully acknowledge conversations with Dr. William |
| 832 |
> |
F. Schneider and Dr. Feng Tao. Support for this project was |
| 833 |
> |
provided by the National Science Foundation under grant CHE-0848243 |
| 834 |
> |
and by the Center for Sustainable Energy at Notre Dame |
| 835 |
> |
(cSEND). Computational time was provided by the Center for Research |
| 836 |
> |
Computing (CRC) at the University of Notre Dame. |
| 837 |
|
\end{acknowledgement} |
| 838 |
|
\newpage |
| 839 |
|
\bibliography{firstTryBibliography} |
| 840 |
|
%\end{doublespace} |
| 841 |
|
|
| 842 |
|
\begin{tocentry} |
| 843 |
< |
%\includegraphics[height=3.5cm]{timelapse} |
| 843 |
> |
A reconstructed Pt(557) surface after having been exposed to a dosage of CO equivalent to half a monolayer of coverage is observed to undergo a doubling of the step-edge height and terrace length. |
| 844 |
> |
\includegraphics[height=3.5cm]{TOC_doubleLayer} |
| 845 |
|
\end{tocentry} |
| 846 |
|
|
| 847 |
|
\end{document} |
